An RTM method which is a so-called “RTM multi-point injection method” is known wherein a preform made of a reinforcing fiber base material is disposed in a cavity of a mold, and a resin is injected from a plurality of resin injection ports facing one surface of the preform to impregnate the resin into the reinforcing fiber base material of the preform (for example, JP-A-2010-89501). Further, an RTM method which is a so-called “line injection method” is also known wherein a resin injection line extending along an end surface of a preform is provided at an end surface side of the preform, and a resin is supplied and injected from the end surface of the preform disposed in a cavity.
In the above-described multi-point injection method, as compared to the line injection method, because the resin can be spread in all directions from each of many injection points disposed on one surface of the preform, it has an advantage excellent in high-speed impregnation property. Further, since injection points are disposed on one surface of the preform, without being greatly influenced by a peculiar flow such as a race-tracking (a phenomenon wherein resins having flowed from different directions regulate their flows to each other and a stripe-like residence portion is formed), that is liable to occur at a position around a preform, the injected resin from each of the respective injection points can be spread in a flow front shape (a shape of a tip portion of a resin flow) in accordance with the resistance of the preform or in a flow front shape corresponding to the shape of an injection port (for example, in the case of a circular injection port, a constant-thickness preform or a quasi-isotropic laminated preform, a circular flow front shape is likely formed). However, because it is a method of injecting a resin into a closed mold, an air trap is liable to occur within a region of a product to be molded, but, for such a case, a method of assisting the resin injection and impregnation by vacuum suction is also introduced.
On the other hand, in the line injection method, because resin is injected from a predetermined resin injection line in a specified direction, the resin injection speed can be controlled relatively easily, and the position of a flow front and the degree of the progress thereof can be monitored also easily. However, if the line of the shape of the flow front does not progress in a desired shape or if there occurs a place hardly reached with the flow front, a portion lacking in impregnated resin (resin starving portion or resin non-impregnated portion) is liable to be generated, and further, there is a fear that a portion which is not sufficiently favorable in surface quality is generated.
However, even in any of the above-described multi-point injection method and line injection method, a race-tracking is still liable to occur around a preform, and there remains a fear that resin first flows around the outer circumferential portion of a preform and air is trapped in a product. If air is trapped in a product, a resin starving portion or a resin non-impregnated portion is liable to be generated and, further, there is a fear that a portion which is not sufficiently favorable in surface quality is generated.
For example, in the above-described RTM multi-point injection method, there is a case where the following problem may occur. The flow front shape depending upon the resistance of a preform or the shape of a resin injection port does not always coincide with the shape of a molded product to be molded. Therefore, there is a case where an air trap is caused within the product by race-tracking and the like. Further, in the method of assisting by vacuum suction, frequently a sufficient time cannot be taken to completely create a vacuum condition, from the balance with a molding cycle time. Therefore, in particular, in the case of a large mold (cavity), there is a case where the molding finishes at a condition where air is finally trapped. Furthermore, as a method of reducing air traps, although there is a method of sucking and evacuating air and resin together (because it is difficult to separate them, as the resin contains bubbles), cleaning an evacuation port is troublesome and, therefore, it involves another problem.
Accordingly, paying attention to the above-described limits in conventional technologies, there is a need to provide a method of producing FRP using RTM method, capable of obtaining a molded product favorable in quality with no air traps, in particular, capable of obtaining a molded product with no air traps by using a completely closed mold.